WHO scientists have said the herbicide 2-4,D shows some characteristics that could be linked to cancer.
Photograph: Brian Brown / Alamy/Alamy

The widely used farm chemical known as 2,4-D – a key ingredient in a new herbicide developed by the Dow company – “possibly” causes cancer in humans, a World Health Organisation research unit has said.

The classification of the weed killer, 2,4-dichlorophenoxyacetic acid, was made by the WHO’s International Agency for Research on Cancer (IARC).

The IARC said it reviewed the latest scientific literature and decided to classify 2,4-D as “possibly carcinogenic to humans”, a step below the more definitive “probably carcinogenic” category but two steps above the “probably not carcinogenic” category.

IARC’s findings on 2,4-D have been awaited by environmental and consumer groups that are lobbying US regulators to tightly restrict the use of 2,4-D. Farm groups and others, meanwhile, defend 2,4-D as an important agent in food production that does not need more restrictions.

In March IARC said it had found another popular herbicide – glyphosate – was “probably carcinogenic to humans”. Glyphosate is the key ingredient in Monsanto Co’s Roundup herbicide and other similar products and brands.

IARC classifications do not carry regulatory requirements but can influence regulators, lawmakers and the public. Following the glyphosate classification some companies and government officials moved to limit its use.

Dow AgroSciences, a unit of Dow Chemical Co, has had a particular interest in IARC’s review. The company is using both glyphosate and 2,4-D in a herbicide it calls Enlist Duo that received US approval last year. Enlist Duo is designed to be used with genetically engineered, herbicide-tolerant crops developed by Dow.

Dow had no comment on the IARC classification but the company has said 2,4-D is a safe and valuable tool for agriculture.

IARC said it decided on the “possibly” classification because there was “inadequate evidence in humans and limited evidence in experimental animals” of ties between 2,4-D and cancer. It said that epidemiological studies provided “strong evidence that 2,4-D induces oxidative stress … and moderate evidence that 2,4-D causes immunosuppression”.

However, IARC said, “epidemiological studies did not find strong or consistent increases in risk of NHL (non-Hodgkin lymphoma) or other cancers in relation to 2,4-D exposure”.

Dana Loomis, a deputy section head for IARC, said the most important studies reviewed showed mixed results and a “sizeable minority” judged the evidence as stronger than others did.

Among the research presented to IARC was an analysis funded by a Dow-backed group that found no ties between 2,4-D and many cancers.

Some critics of 2,4-D had thought IARC would assign at least the “probably” cancer-causing classification to 2,4-D. But the Dow-backed 2,4-D Research Task Force said there was no reason to do so: “Not one health and safety regulator in the world consider 2,4-D to be a human carcinogen,” it said in a statement.

Since its introduction in 1945 2,4-D has been widely used to control weeds in agriculture, forestry, and urban and residential settings.